People with spinal cord injury (SCI) experience uncontrolled involuntary muscle contractions (spasticity) that disrupt mobility, sleep, and comfort. Spasticity is typically treated pharmacologically, with myriad side effects. A more obvious consequence of SCI is impaired voluntary muscle control, of which limitations of walking function are one of the most devastating consequences. Numerous clinical trials have attempted to address these issues, however, to date no studies have undertaken a systematic approach that involves identify dose-response relationships, in the manner of pharmaceutical trials, prior to the intervention. Afferent input in the form of vibration has long been known to have a powerful effect on neural circuits; whole body vibration (WBV) represents an intense and generalized form of this afferent input. Preliminary studies from our lab suggest that afferent input in the form WBV may reduce spasticity improve walking and persons with SCI. Our findings related to the influence of WBV on spasticity are supported by a long history of studies showing that local vibration activates modulatory mechanisms. Recent studies from our lab have shown that local vibration elicits involuntary step-like behavior, even in persons with motor-complete SCI. Much of the prior work of our lab has centered on the use of afferent input, often combined with repetitive task practice, to promote neuroplasticity and improved motor function in persons with SCI. However, we have never been sure that the dose of afferent input (or the dose of practice) we used was optimal. This is a common problem that plagues rehabilitation research. Questions of dose are needed before progressing to larger trials. There is a clear need for rehabilitation research studies that are developed in progressive stages in the manner of pharmaceutical trials. Accordingly, the proposed study is a 2-Phase development-of-concept trial intended to evaluate dose-response effects, identify the most suitable outcome measures, and select optimal components of the intervention. This study will provide answers related to dose effects of WBV on spasticity, walking function, pain, and muscle strength by focusing on the following Aims: Specific Aim 1. (Phase 1; Years 1 - 2) Quantify early and late within-session effects on spasticity and walking function of 5 different WBV doses (4 doses, plus placebo) delivered 1 week apart. Specific Aim 2. Quantify the cumulative and persistent dosing effects on spasticity of once daily (1xWBV) versus twice daily (2xWBV) WBV at the dose most effective for reducing spasticity. Specific Aim 3 (Phase 2; Years 3 - 5). Quantify the cumulative and persistent dosing effects on walking function of 1xWBV versus 2xWBV at the most effective dose. Specific Aim 4 (Phase 2; Years 3 - 5). Characterize the cumulative and persistent dosing effects on pain and leg strength of 1xWBV versus 2xWBV at the most effective dose.